Severe acute respiratory syndrome (SARS) is a respiratory illness that has recently been reported in Asia, North America, and Europe. In general, SARS is being noticed with a fever greater than 100.4° F. [>38.0° C.]. Other symptoms may include headache, body aches, and an overall feeling of discomfort. Some people also experience mild respiratory symptoms. After 2 to 7 days, SARS patients may develop a dry cough and have trouble breathing with the symptom of atypical pneumonia.
The primary way that SARS appears to spread is by close person-to-person contact. Most cases of SARS have involved people who cared for or lived with someone with SARS, or had direct contact with infectious material (for example, respiratory secretions, stool and others) from a person who has SARS. Potential ways in which SARS can be spread include touching the skin of other people or objects that are contaminated with infectious droplets and then touching victim's eye(s), nose, or mouth. This can happen when someone who is sick with SARS coughs or sneezes droplets onto themselves, other people, or nearby surfaces. It also is possible that SARS can be spread more broadly through the air or by other ways that are currently not known.
Scientists studying the etiology of SARS have detected a previously unrecognized coronavirus in patients with SARS. The new SARS coronavirus (SARS-CV) is the leading hypothesis for the cause of SARS. Initial diagnostic testing for suspected SARS patients should include chest radiograph, pulse goniometry, blood cultures, sputum Gram's stain and culture, and testing for various viral respiratory pathogens including influenza A and B and respiratory syncytial virus. Clinicians should save any available clinical specimens (respiratory, blood, and serum) for additional testing until a specific diagnosis is made. Acute and convalescent (greater than 21 days after onset of symptoms) serum samples should be collected from each patient who meets the SARS case definition. Paired sera and other clinical specimens should be collected for advanced diagnosis.
According to the expression “Severe Acute Respiratory Syndrome (SARS): Laboratory diagnostic tests” described on 29 Apr. 2003 from WHO website, researchers in several countries are working towards developing fast and accurate laboratory diagnostic tests for the SARS coronavirus (SARS-CV). However, until standardized reagents for virus and antibodies detection are available and methods have been adequately field tested, SARS diagnosis remains based on the clinical and epidemiological findings: acute febrile illness with respiratory symptoms not attributed to another cause and a history of exposure to a suspect or probable case of SARS or their respiratory secretions and other bodily fluids.
According to the Centers for Disease Control and Prevention (CDC), “Initial diagnostic testing for suspected SARS patients should include chest radiograph, pulse oximetry, blood cultures, sputum Gram's stain and culture, and testing for viral respiratory pathogens, notably influenza A and B and respiratory syncytial virus. A specimen for Legionella and pneumococcal urinary antigen testing should also be considered.”
In general, there are three current methods for detecting SARS-CV. That is, molecular tests, antibody tests and cell culture.
Although molecular tests such as polymerase chain reaction (PCR) can detect genetic material of the SARS-CV in various specimens (blood, stool, respiratory secretions or body tissues sampling for SARS diagnostic tests), existing PCR tests are very specific but lack sensitivity. This means that negative tests cannot rule out the presence of the SARS virus in patients. Furthermore, contamination of samples in laboratories in the absence of laboratory quality control can lead to false positive results. In addition, negative PCR results do not exclude SARS. SARS-CV PCR can be negative for the following reasons: (1) the patient is not infected with the SARS coronavirus; the illness is due to another infectious agent (virus, bacterium, fungus) or a non-infectious cause; and (2) the test results are incorrect (“false-negative”). Current tests need to be further developed to improve sensitivity. Specimens were not collected at a time when the virus or its genetic material was present. The virus and its genetic material may be present for a brief period only, depending on the type of specimen tested.
Antibody tests detect antibodies produced in response to the SARS coronavirus infection. Different types of antibodies (IgM and IgG) appear and change in level during the course of infection. They can be undetectable at the early stage of infection. IgG usually remains detectable after resolution of the illness. However, the test formats (such as ELISA (Enzyme Linked ImmunoSorbant Assay) and IFA (Immunofluorescence Assay) are being developed. Further, although some patients have detectable coronavirus antibody within 14 days of illness onset, definitive interpretation of negative coronavirus antibody tests is possible only for specimens obtained >21 days after onset of fever.
To monitor the explosive invasiveness of SARS-CV, an early diagnosis of virus itself would be essential to solve the urgency of detection of SARS-CV. Most of time, viral pathogens are able to down-regulate the host immune system that may end up with a low grade or delayed immune response. Therefore, host antibody couldn't be the good surrogate marker for diagnosis.
The presence of the infectious virus can be detected by inoculating suitable cell cultures (e.g., Vero cells) with patient specimens (such as respiratory secretions, blood or stool) and propagating the virus in vitro. Once isolated, the virus must be identified as SARS-CV using further tests. Cell culture is a very demanding test, but currently (with the exception of animal trials) only means to show the existence of a live virus. It has to be performed under at least biosafety safety level (BSL) 3 conditions. Positive cell culture results indicate the presence of live SARS-CV in the sample tested. However, negative cell culture results do not exclude SARS as the similar reason as negative PCR test result.
All tests for SARS-CV available so far have limitations. Extreme caution is therefore necessary when management decisions are to be based on virological test results. In particular, false negative test results (due to low sensitivity, unsuitable sample type, or time of sampling, etc.) may give a false sense of security; in the worst case, they could allow persons carrying the SARS virus, and therefore capable of infecting others, to escape detection.
In addition to allowing the rapid diagnosis of SARS infection, the availability of diagnostic tests will help to address important questions such as the period of virus shedding (and communicability) during convalescence, the presence of virus in different body fluids and excreta, and the presence of virus shedding during the incubation period.
Appropriate specimen should be collected and analyzed upon the timing and severity of disease. Diagnostic samples should be suitable for viral culture, PCR, antigen detection, immunostaining and/or serological antibody assays. In general, blood, sputum, throat swab and stools are frequently studied in case of SARS. For unknown reason, it seldom uses urine to detect or assay SARS-CV.
Given the above, current available assay cannot quickly and completely detect SARS-CV. It requires a quick assay with high specificity and sensitivity to detect SARS-CV from the easily available samples (such as urine).